Posterior chamber lens implants consist of a lens, typically made of polymethylmethacrylate (PMMA), and haptic portions projecting from the lens periphery. The haptic portions hold the lens in position in the eye, within the capsular bag between the posterior capsule and the iris, after the implant has been inserted through an incision at the junction of the cornea and the sclera. The haptics are in the form of closed loops or curved arms, which lie generally in the plane of the lens.
During implantation, a posterior chamber lens is inserted into the posterior chamber through the pupil. In order to pass through the pupil, the loops must be compressed, since the pupil cross-section is relatively narrow. Once the implant is in the larger, posterior chamber, it is necessary that the loops spring open again so as to press outwardly against the interior of the posterior chamber to seat and hold the lens insert in place.
Because posterior chamber implants require such highly flexible loops, in the past the loops for such implants were made of Prolene rather than PMMA. Prolene loops can be made relatively thin and highly flexible and resilient to facilitate insertion through the pupil. Moreover, Prolene loops, when implanted, do not press with excessive force against the interior surface of the eye. However, Prolene is subject to degradation in the eye, so attempts were made to use PMMA loops. Because of the relative properties of PMMA, the substitution of PMMA resulted in loops which were too stiff, and thus unable to bend sufficiently, or which exerted excessive outward force, or which, if made too thin, were subject to breaking.
My U.S. Pat. No. 4,476,591 discloses a lens implant with holding loops or arms having a geometry in which the loops are made of PMMA but are nevertheless highly flexible and, when implanted, hold the lens without exerting excessive force against the interior surface of the eye. According to this lens design, the loops project radially outwardly from the lens, and then bend sharply and extend around the lens a distance so as to overlap one another, i.e. to encircle the lens. Not only does this lens possess highly flexible loops, but the fact that the loops completely encircle the lens enhances the centering properties of the loops and forms a protective barrier to inhibit the growth of secondary fibers inwardly over the posterior capsule.
In the '591 implant, each loop includes a first portion that extends substantially radially outwards from the lens, a sharp bend, a second loop portion with a curvature that follows the lens circumference, a third loop portion that extends from the second loop portion and has a curvature less than the immediately preceding loop portion, and a fourth loop portion that extends from the third loop portion and has a greater curvature than the third loop portion, e.g. substantially similar to that of the second portion. In this manner, the loops, when compressed toward one another, as occurs when the lens is implanted in the eye, maintain their encircling configuration and bend into a more circular shape. The encircling loops provide a large contact area between the loops and the interior surface of the eye to distribute the holding force over a greater area. Also, since secondary fibers tend eventually to grow around the loops and hold them in place, the larger contact area promotes long term stability of the lens in the eye.
Another advantage of this lens configuration occurs during insertion of the lens. Posterior chamber lenses are implanted by forming an incision in the cornea and inserting the lens axially through the incision. When the lens is oriented for insertion, the stiff first loop portions are substantially axially aligned, i.e. project axially rather than laterally from the lens proper. This means that a smaller incision may be used.
Because the end of each loop overlaps the other loop, care must be taken, when compressing the loops during implantation, to ensure that the free end of one loop does not become entangled with the other loop. If this were to occur, it would be necessary to free the caught end so that the loops could spring outwardly and engage the inner circumferential walls of the capsular bag into which the lens is implanted. It would, however, be desirable to eliminate this possible complication while retaining the substantial benefits of my prior lens insert.